The question of realism is a vexed one in roleplaying circles. Different people suspend their disbelief at different stages. None of my current group raised an eyebrow when the sorcerer fired bolts of lightning. However once they reached space they suddenly started pointing out that electricity can’t pass through a vacuum.
Hit points are a widely accepted implausibility.
Although there are different systems for precisely how much punishment characters can take (some like D&D subscribe to critical existence failure while others, like Shadowrun, render the character increasingly crippled) there are three principle forms of justification for such durability, mapping to a famous three fold classification of gaming styles:
Gamist: It wouldn’t be much fun if your beloved character was crippled or killed in seconds would it? This justification appeals to Ben here.
Simulist: Characters of the sort you are playing as really are that tough. If you are playing as deities (like in Exalted or Scion), werewolves and the like this makes sense. If you are playing as highly trained, but human, spies it makes less sense.
Narrativist: Characters shouldn’t die unless and until their death is meaningful. This is most explicit in 7th Sea where characters can only die under specific conditions selected during character generation but informs other games. Dying to defeat the greater demon who destroyed the Old Kingdoms? Awesome. Tripping on a rug and getting run over by a applecart? Less awesome.